A relay is used between a transmitter and receiver when the received signals at the receiver otherwise should be too weak. When the term relay is mentioned it can either be an amplify-and-forward (AF) relay which is sometimes called a repeater or it can be a decode-and-forward (DF) relay. In the literature one can also find relays with functionality in-between AF and DF and one such example is an Estimate and Forward (EF) relay.
A known problem with relays is the self-interference caused by poor isolation between the output and input of the relay. The self-interference will heavily limit the performance of a relay. For example, the amplification gain of an AF relay and the detection performance of a DF relay are both limited by the isolation. For a relay to properly serve its purpose, which is to increase the end-to-end (E2E) performance, it is of importance that the isolation problem is solved.
Existing technologies for relay self-interference mitigation include e.g:                Half-duplex operation, either Time Division Duplex (TDD) or Frequency Division Duplex (FDD) and        Orthogonal receive and transmit antenna polarizations.        
One way to eliminate the self-interference is to operate the relay in half-duplex mode. That is, the relay is not receiving and transmitting at the same time, TDD, or at the same frequency, FDD, and thus has infinite isolation between input and output. Half-duplex operation, however, incurs a capacity loss, of 50% usually. This dramatic loss of capacity is not acceptable and other solutions for increasing the isolation are needed. One such solution for an on-frequency relay (on-frequency operation means that the relay receives and forwards on the same frequency) is to use antenna techniques such as e.g. orthogonal polarizations on receive and transmit sides. However, since the polarization of the user equipment (UE) most likely is unknown it might therefore be desirable to transmit the forwarded signals over two orthogonal polarizations in order to prevent polarization mismatch between relay and UE. Thus, one might not want to sacrifice one of the polarizations for self-interference reduction.
There is thus a need to suppress self-interference between the output and input of the relay without the drawbacks described above of reduced capacity of half-duplex operation or use of orthogonal polarizations.